1. Field of the Invention
The present invention relates generally to manipulating fluid flow over a surface. Particularly, the present invention relates to actively and passively manipulating fluid flow over an aerodynamic or hydrodynamic surface. More specifically, the present invention relates to methods and systems of providing distributed flow control actuation to manage the behavior of a global flow field.
2. Description of the Related Art
Affordability is becoming the dominant design requirement for future tactical aircraft. Affordability must be achieved, however, while simultaneously improving both survivability and aerodynamic performance. To meet vehicle affordability goals, future propulsion systems must be lighter, more compact, and must accommodate ever-increasing integration between the air vehicle, engine, and various subsystems. The engine inlet system shares these goals. Inlet duct design parameters such as the offset, wall curvature rate, shaping, diffusion rate, etc., however, are limited by considerations of pressure loss and flow non-uniformity, i.e., distortion due to turbulence resulting from the shape and wall curvature rate of the of the inlet duct. As future systems evolve toward more compact designs with exotic, survivability-driven shaping, these limitations will in turn limit the design space for the vehicle itself. A need thus continues to exist for new technologies that can overcome these inlet design limitations.
One of the most commonly used methods to control local boundary layer separation within ducted systems is the placement of vortex generators upstream of the layer separation within a natural fluid flow. Vortex generators are small wing like sections mounted on the inside surface of the ducted fluid flow and inclined at an angle to the fluid flow to generate a shed vortex. The height chosen for the best interaction between the boundary layer and the vortex generator has previously been the boundary layer thickness. The principle of boundary layer control by vortex generation relies on induced mixing between the primary fluid flow and the secondary fluid flow. The mixing is promoted by vortices trailing longitudinally near the edge of the boundary layer. Fluid particles with high momentum in the stream direction are swept along helical paths toward the duct surface to mix with and, to some extent, replace low momentum boundary layer flow. This is a continuous process that provides a source to counter the natural growth of the boundary layer creating adverse pressure gradients and low energy secondary flow accumulation.
Application of such local flow control methods to advanced serpentine inlet ducts, however, has been found to be inadequate in achieving a sufficient reduction of engine face distortion. The “local” use of vortex generators generally only allows separation to be controlled at one flow condition (usually the cruise condition), with all other conditions rendered “off-design.” Functional implementations generally include application of a single row of ten or so vortex generators near the point of incipient separation. Although providing an improvement in the amount of total pressure loss, generally on the order of one to two percent maximum, such generators generally result in a minimum parasitic drag of between one-half of one percent to one percent. Further, when implemented in the form of vanes, such sizing generally limits the angle of attack to approximately twelve degrees maximum.
More recent applications, such as those described in U.S. Pat. No. 6,371,414 titled “System and Method for Manipulating and Controlling Fluid Flow over a Surface” and in Hamstra et al. “ICAS-2000-6.11.2 Active Inlet Flow Control Technology Demonstration” presented at the 22nd International Congress of Aeronautical Sciences, 27 Oct.-1 Sep. 2000, Harrogate, United Kingdom, each incorporated by reference in its entirety, have taken a more global approach, utilizing multiple sets of micro-vanes and micro-jets sized down to as low as one-tenth of the thickness of the boundary layer, and positioned near each separate point of incipient separation. Although substantially increasing performance, there nevertheless remains a continuing need to further reduce the maximum total pressure loss and the minimum expected parasitic drag.
Accordingly, the inventors have recognized the need for methods and systems to provide distributed flow control actuation to manage the behavior of a global flow field, which can achieve macroscopic effects by manipulating nanoscopic conditions, and which can provide a substantial reduction in pressure loss and parasitic drag over that of prior systems. Also recognized is the need for a control system in conjunction with a separate multi-dimensional array of nanoscopic actuators positioned at strategic locations with respect to each separate expected point of incipient separation of the flow.